[填空题]The moon is a very silent world, for sound waves can only travel through the air.

[单项选择] Under proper conditions, sound waves will be reflected from a hillside or other such obstruction. Sound travels at the rate of about one-fifth of a mile per second. If the hill is eleven hundred feet away, it takes two seconds for the sound to travel to the hill and back. Thus, by timing the interval between a sound and its reflection (the echo), you can estimate the distance to an obstruction. During World War I1 the British used a practical application of this principle to detect German planes on their way to bomb London before the enemy was near the target. They used radio waves instead of sound waves, since radio waves can penetrate fog and clouds. The outnumbered Royal Air Force (RAF) always seemed to the puzzled Germans to be lying in wait at the right time and never to be surprised (袭击). It was radio echoes more than anything else that won the Battle of Britain. Since the radio waves were used to tell the direction in which to send the RAF planes and distance to send
A. height of the hill
B. speed of sound
C. distance of the hill
D. intensity of sound

[填空题]Very high waves are destructive when they S36 ______ the land. Fortunately, this S37 ______ happens. One reason is that out at sea, waves moving in one direction almost always run into waves moving in a different direction. The two S38 ______ of waves tend to cancel each other out. Another reason is that water is S39 ______ near the shore. As a wave gets closer to land, the shallow S40 ______ helps reduce its S41 ______ . S42 ______ , waves sometimes strike the shore with the force of 6,000 pounds for each square foot. That means a wave, 25 feet high and 500 feet along its face, may S43 ______ . Yet S44 ______ . During the most raging storms, S45 ______.S37